Stent and method and apparatus for forming and delivering the same

ABSTRACT

A stent comprising a wire skeletal frame, the frame being adapted to assume a first condition in which the frame is relatively rigid and substantially tubular in configuration and a second condition in which the frame is flexible, of reduced stress, and collapsible, such that in the second condition walls of the frame are adapted to be positioned against each other to form a stent diameter substantially equal to the combined thickness of the frame walls in abutting engagement with each other, the frame in its second condition being substantially devoid of bias therein urging the frame to assume the first configuration.

This application is a divisional application of application Ser. No.08/649,289 filed May 17, 1996, now U.S. Pat. No. 5,746,765 which is adivisional application of U.S. Ser. No. 08/252,198 filed Jun. 1, 1994,now U.S. Pat. No. 5,540,712.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to stents and is directed more particularly to aself-expanding stent which is repositionable after being set in place.

2. Brief Description of the Prior Art

Self-expanding stents are generally known in the art. U.S. Pat. No.4,580,568, issued Apr. 8, 1986, to Cesare Gianturco, discloses anendovascular stent formed of stainless steel wire. The stent iscompressed into a reduced size having an outer diameter substantiallysmaller than the stent in its expanded shape. The stent is held in itscompressed state during its passage through a small bore catheter untildelivered into a vascular system passageway, whereupon the stress in thestent causes the stent to expand in the larger bore vascular passagewayto hold open the passageway. When the stent is compressed, the bends inthe wire, which is of a zig-zag configuration, store stress, and thestent is expandable by the release of the stress stored in the bends.Once set in place, the radial extremities of the stent bear against theinside walls of the passageway. There is no ready means by which thestent may be again compressed, or softened, so that the stent may berepositioned.

It would be beneficial to the medical arts to have available a stentadapted for compression into a small size to facilitate introductioninto a vascular passageway, and adapted for self-expansion in thevascular passageway to hold open the passageway, and also adapted to besoftened and/or contracted to permit repositioning of the stent.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide a stent adaptedto assume a first configuration in which the stent is expanded, capableof exercising considerable stress if confined, as by a vessel wall, andsubstantially tubular in configuration for holding open a vascularpassageway, and a second configuration in which the stent is flexible,in a reduced stress state, and adapted to be compressed into a smallenough size to fit within the small bore of a delivery catheter.

A further object of the invention is to provide such a stent which isadapted to change from the first condition of relative rigidity to thesecond condition of flexibility and reduced stress, by exposure to apreselected transition temperature, such that the stent may be relaxedin place in a vascular passageway by cooling to facilitate repositioningthereof without damage to walls of the passageway.

A still further object of the invention is to provide such a stentlaminated within an elastomeric sleeve, the sleeve being expandable toconform to the stent's first, i.e. rigid, condition and having therein abias towards assuming a smaller size, such that upon the stent'sassuming the second, i.e. flexible, condition, the sleeve operates tocompress the stent to a size less than its expanded size.

Yet another object of the present invention is to provide such a stentformed from a plurality of cells each having first and second spaced,substantially parallel cell sides which are joined to one of the firstor second cell sides of an adjacent cell. The cell sides of all of thecells are substantially parallel to the central longitudinal axis of thestent or the stent section which the cells form.

A further object of the present invention is to provide such a stentformed from a plurality of cells, each of which includes first andsecond spaced, straight side portions which are joined to one of thefirst or second straight side portions of an adjacent cell. The straightside portions of all cells are substantially parallel to the centrallongitudinal axis of the stent or the stent section which the cellsform. The ends of each cell are closed by end portions which extendbetween the adjacent ends of each side portion at an angle to thecentral longitudinal axis of the stent or the stent section which thecells form. These end portions are not connected to adjacent cells.

A further object of the present invention is to provide such a stentformed from stent sections having different structural configurationsand/or which provide different amounts of outward radial force when thestent sections are expanded from a collapsed disposition.

A further object of this invention is to provide a stent delivery unitwhich facilitates anchoring of the proximal end of the stent in placebefore expansion of the distal end.

A still further object of the present invention is to provide a noveland improved method and apparatus for forming a stent of thermal memorymaterial wherein the cells forming the stent are welded before the stentis heat treated to thermally determine the shape memory for the stent.

With the above and other objects in view, as will hereinafter appear, afeature of the present invention is the provision of a stent comprisinga wire skeletal frame, the frame being adapted to assume a firstcondition in which the frame is expanded and resiliently deformable butrelatively rigid, the frame being further adapted to assume a secondcondition in which the frame is flexible, of reduced stress andcollapsible, such that in the second condition walls of the frame areadapted to be positioned in their collapsed disposition, and furtheradapted to be positioned against each other to form a stent diametersubstantially equal to the combined thickness of the frame walls inabutting engagement with each other, and further adapted to bepositioned between the expanded disposition and the walls abuttingengagement disposition, the frame in the second condition beingsubstantially devoid of bias present therein urging the frame to assumethe first configuration.

In accordance with a further feature of the invention, there is provideda stent, as described immediately above, and further comprising anelastomeric sleeve disposed on the stent and expandable therewith toconform to the stent's expanded condition, the sleeve having therein abias exerting a compressive force on the stent, such that upon coolingof the stent below a selected transition temperature, the sleeve urgesthe flexible and low stress stent to a third configuration smaller thanthe stent in its expanded condition and larger than the stent in itswalls abutting configuration.

The above and other features of the invention, including various noveldetails of construction and combinations of parts will now be moreparticularly described with reference to the accompanying drawings andpointed out in the claims. It will be understood that the particulardevices embodying the invention are shown by way of illustration onlyand not as limitations of the invention. The principles and features ofthis invention may be employed in various and numerous embodimentswithout departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made to the accompanying drawings in which are shownillustrative embodiments of the invention, from which its novel featuresand advantages will be apparent.

In the drawings:

FIG. 1 is a perspective view of one form of stent illustrative of anembodiment of the invention;

FIG. 2 is a side elevational view thereof;

FIG. 3 is a side elevational view of an alternative embodiment thereof;

FIG. 4 is a side elevational view of a second alternative embodimentthereof;

FIG. 5 is a side elevational view of the stent shown in FIG. 1, butshown in a compressed condition;

FIG. 6 is a side elevational view of the stent shown in FIGS. 1 and 2with an elastomeric sleeve thereon;

FIGS. 7A-7C are illustrative stylized diagrammatic views of one mannerof use of the inventive devices of FIGS. 1-6, as in the treatment of ananeurysm of a large artery;

FIGS. 8A-8C are stylized diagrammatic views illustrative of anothermanner of use of the inventive device of FIGS. 1-6, as in the treatmentof compression or narrowing of a vessel;

FIGS. 9A-9E are stylized diagrammatic views illustrative of a manner ofrepositioning the inventive device of FIGS. 1-6;

FIG. 10 is a side elevational view of a third embodiment of the stent ofthe present invention;

FIG. 11 is a side elevational view of a fourth embodiment of the stentof the present invention;

FIG. 12 is a side elevational view of a fifth embodiment of the stent ofthe present invention;

FIG. 13 is a block diagram showing the mechanism used to manufacture thestent of the present invention;

FIG. 14 is a sectional exploded view of a stent delivery unit of thepresent invention; and

FIGS. 15A-15E are sectional views illustrating the manner in which thestent delivery unit of FIG. 14 positions a stent.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, it will be seen that an illustrative stentincludes a skeletal frame 2, preferably formed from a single wire 4. Thewire 4 includes a plurality of abutting straight portions 6 which arejoined to each other, as by welding.

In FIGS. 1 and 2, the illustrative stent is shown in a first conditionin which the frame 2 is expanded, relatively rigid, and substantiallytubular in configuration. Ends 8, 10 of the single wire 4 are disposedin one of the welded straight portions 6, such that there are no exposedwire free ends, disposed within or extending from the frame 2. Theabutting and elongated straight portions 6 of the wire 4 facilitate theuse of strong elongated welds to securely join the wire portions 6together. The wire 4 preferably is round in cross-section, but may beformed of any desired cross-sectional shape. In the frame straightportions 6 the joined wire segments are disposed, relative to thetubular configuration of the frame, circumferentially thereof. The wire4 abuts itself only at the straight portions 6 and does not cross itselfat any point. Accordingly, the frame walls, that is, walls 12 of atubular body portion 14 of the frame 2 have a thickness equal to thediameter of the wire 4.

The stent includes the body portion 14 and finger portions 16 extendinggenerally axially from one, or both, ends of the body portion. Thefingers facilitate a gradual reduction in radially outwardly extendingpressure exerted by the stent on the wall of a vascular passageway inwhich the stent is located. Such gradual reduction of pressurefacilities acceptance of the stent by the passageway and reducesdeleterious reactions by the passageway wall to the presence of thestent. Referring to FIG. 3, it will be seen that the finger portion 16may be extended further axially to lessen the probability of adversereaction by the passageway wall to the pressure exerted against the wallby the stent frame 2. Also, hooks or barbs 17 can be attached to projectoutwardly from some of the straight portions 6 to aid in anchoring thestent to the vessel in which it is implanted. These hooks can be formedfrom Nitinol in either its memory form or superelastic form, or they canbe formed from other material such as biologically resorbable polymers.

The tubular body portion 14 comprises a mesh formed by the wire 4, themesh comprising a plurality of interconnected cells 18 which arepreferably of a polygonal configuration when viewed in plan, providingspaced, substantially parallel straight sides to form the aforementionedstraight portions 6. The cells 18, when polygonal, are preferably of ahexagonal configuration, which readily provides expansion and rigiditycharacteristics desirable in the structure and operation of the device.Preferably, the stent comprises six of the polygonal cells 18circumferentially and an even number of the polygonal cells along itslength, thereby facilitating formation of the stent by the single wire4. The portion of the stent having the mesh construction exercises asubstantially greater radial bias than do the finger portions 16. Thus,when it is desired to have more force near the ends of the stent than atits center, the embodiment shown in FIG. 4 may be used. Referring toFIG. 4, it will be seen that in this embodiment, the central portion ofthe tubular body portion 14 includes elongated cells 20 exercising lessradial force than the cells 18.

It is important to note that each cell is formed by two straightportions 6 which are substantially parallel to the central longitudinalaxis of the stent or stent section of which the cell is a part. Each endof the cell is closed by an end wall or end walls 21 which extendbetween adjacent ends of the straight portions 6; the end walls beingdisposed at an angle to the central longitudinal axis of the stent orstent section containing the cell.

The stent preferably is made of an alloy of nickel and titanium whichprovides the stent with a thermal memory. The unique characteristic ofthis alloy, known generally as "Nitinol", is its thermally triggeredshape memory, which allows the stent constructed of the alloy to becooled below a temperature transformation level and thereby softened forloading into a catheter in a relatively compressed and elongated state,and regain the memoried shape when warmed to a selected temperature,above the temperature transformation level, such as human bodytemperature. The two interchangeable shapes are possible because of thetwo distinct micro-crystalline structures that are interchangeable witha small variation in temperature. The temperature at which the stentassumes its first configuration may be varied within wide limits bychanging the composition of the alloy. Thus, while for human use thealloy may be focused on a temperature of 98.6° F. for assumption of thefirst condition, the alloy readily may be modified for use in animalswith different body temperatures.

Although the stents shown in FIGS. 1-4 are preferably formed of wire,they could be formed from a sheet of Nitinol which has been stamped toform the cells 18. Once the cells are formed, the opposed longitudinaledges of the sheet would be joined to form the frame 2 with the straightportions 6 oriented in parallel relationship to the longitudinal axis ofthe stent.

Accordingly, when the stents shown in FIGS. 1-4 are subjected to atemperature at or less than the transition temperature, the relativityrigid stent changes to a second condition in which it is flexible, ofreduced stress and collapsible. The stent does not, of its own accord,collapse, or compress, but the stent does become quite pliable,collapsible and compressible. By mechanical means, the stent may becompressed to a point at which the walls 12 of the body portion 14 ofthe stent frame 2 are positioned against each other, to form a stentdiameter substantially equal to the combined thickness of the framewalls in abutting engagement with each other. In FIG. 5, the stent isshown approaching, but not yet having reached such minimal stentdiameter. In the compressed condition, the stent is readily contained bya catheter C (FIG. 7B).

In FIG. 6, there is shown an alternative embodiment having still furtherbenefits. As noted above, in the second condition of the stent, thestent becomes flexible and compressible, but does not of its own accordcompress. In the embodiment shown in FIG. 6, the stent body portion hasdisposed thereon an elastomeric sleeve 22. The sleeve 22 is expandableon the frame 2 as the frame expands to its enlarged configuration.However, as the sleeve expands, the sleeve exerts a compressive force onthe frame. Upon cooling of the stent to or below the transitiontemperature, the stent becomes flexible and the compressive sleeve 22urges the frame 2 to a third configuration of smaller diameter than thefirst configuration. Accordingly, upon cooling of the sleevedembodiment, the flexible frame automatically reduces in size, therebyrendering any repositioning of the stent, as by a grasping tool or otherinstrument, known in the art (not shown), a relatively simple matter.Again, upon removal of the cooling medium, the sleeved stent returns toits expanded condition.

The sleeved stent has an added benefit in that while an unsleeved stentwill suffice in many instances, there are occasions when the affectedpassageway wall is in such a weakened condition that the provision of anew wall, or a graft, is required. The sleeved stent is essentially agraft and operates to provide a new passageway wall when required.

In operation, the stent, sleeved or unsleeved, is carried through anaffected vascular passageway V (FIG. 7A) by the catheter C (FIG. 7B),which is of a thermally insulative material. At room temperature, andwhile cooled by infusion of a cool solution within the catheter, thestent remains in the second condition, flexible and of low stress. Beingof low stress, the stent exercises negligible radial force against theinside wall of the catheter and is easily moved through the catheter atthe appropriate time.

As the catheter enters the passageway V, the thermal insulativeproperties of the catheter and the flow of cool solution maintain thestent at less than body temperature. When the distal end of the catheteris properly disposed, as for example, in the vicinity of an aneurysm A(FIG. 7B), the stent is moved out of the end of the catheter C. As thestent contacts blood flow, and is subjected to body temperature, theexposed stent immediately and rapidly assumes its first condition,expanding against the walls of the passageway. Upon total ejection ofthe stent, the catheter is removed, leaving the stent in place to act asan internal wall graft (FIG. 7C).

Referring to FIGS. 8A-8C, it will be seen that in treatment ofcompression of a large vessel, such as a superior vene cava S, thecatheter C (FIG. 8B) is moved through the vessel S to a point adjacent astricture T. The stent 2 is moved from the catheter C, while thecatheter is withdrawn, to place the emerging stent within the vessel andin the area of the stricture (FIG. 8B). As the stent emerges from thecatheter, the stent, as it is exposed to the blood stream, assumes itsfirst condition. Upon total removal of the stent from the catheter, thestent in its entirety is expanded against the wall of the vessel (FIG.8C) to maintain the vessel in a free-flowing configuration.

The cell structure and orientation within the stent is very important tothe proper expansion and compression characteristics of the stent. Sincecell joinder is accomplished solely at adjoining straight portions 6,the expansion of the stent radially and outwardly from the centrallongitudinal axis or axes thereof places minimal stress on theconnections between cells. The straight portions 6, being parallel tothe longitudinal axis of the stent or stent section, do notsignificantly change in configuration as the stent is collapsed andexpanded.

Since the sole connection between cells is along these straightportions, the connection is not subjected to tension or shear forceduring expansion and compression of the stent in a manner which wouldtend to stress and break the connection. The end walls 21, which areinclined relative to the central longitudinal axis of the stent or stentsection, are the portions of the cell which provide the radial memoryforce during expansion, and the longitudinally oriented connectionsbetween the cells causes the cells to distribute this radial memoryforce evenly around the stent. It is the pliability of the end walls attemperatures below the temperature transformation level which cause thecell straight portions 6 to move together as the stent is compressed,and it is these same end walls which become relatively rigid butresiliently deformable to return the stent to its thermal memory shapeat temperatures above the temperature transformation level. As these endwalls maintain the straight portions 6 of the cells substantiallyparallel to the longitudinal axis of the stent in all configurations ofthe stent, these straight portions are not significantly biased orstressed.

Once expanded in place within a body vessel, a stent is often subjectedto forces which tend to crush the stent within the vessel, and knownstents, once crushed, are not self expanding. For example, a stentplaced in the back of the leg is subjected to forces as the leg bends oris compressed in the seated position of a subject which tend to crushthe stent. The stent of the present invention is self expanding ifcrushed due to the inclined end walls 21. These end walls will flex topermit the straight portions 6 to move inwardly toward the longitudinalaxis of the stent or stent section in response to a crushing force, butwill spring outwardly to their original memory position once thecrushing force is removed.

The ratio of expanded stent diameter to compressed stent diameter can becontrolled within limits by selection of wire diameter. The diameter ofthe expanded stent generally is on the order of 6 to 10 times thediameter of the compressed stent, but can be as great as 20 times thatof the compressed stent. In general, the greater the diameter of thewire 4, the less the ratio of the stent collapsed/expanded diameter. Byselection of wire diameter, it is possible to vary the radial forcewhich the expanded stent will exert on the interior walls of thepassageway in which the stent is set.

It is sometimes the case that once the stent is in place and in partexpanded, it is recognized that the stent is somewhat off target (FIGS.9A and 9B) and requires repositioning. To reposition the stent of thepresent invention, the operator introduces into the passageway a coolmedium M (FIGS. 9C and 9D), such as a saline solution, having atemperature at or less than the transition temperature. When the coolsolution encounters the stent, the stent immediately turns flexible andsurrenders radial force against the passageway walls. In such relaxedstate, tile stent, which has no free wire ends, is easily slid into theproper position by manipulation of the catheter C (FIG. 9D), whereuponthe flow of cool solution is stopped and the stent, upon returning to abody temperature, reassumes its expanded condition in the passageway(FIG. 9E). The catheter C is then withdrawn from the stent and from thepassageway.

Thus, there is provided a stent which may be alloyed to have a selectedtemperature at which the stent assumes its first condition and aselected transition temperature, at which the stent assumes its secondcondition, and which includes a wire frame, wherein the diameter of thewire is selectable to provide a selected degree of expansion force. Thestent is compressible to less than a catheter-size diameter tofacilitate delivery of the stent to a location within a body passagewayby a catheter. The stent may be sleeved or unsleeved. The stent isself-expanding upon delivery from the catheter and introduction to abody temperature, to provide an internal graft or hold open apassageway. Even after such positioning and expansion, the stent isrendered flexible and readily repositionable merely by the flow of acool medium through the stent. And, finally, by termination of the flowof cool fluid, the stent automatically reassumes its passagewaysupporting rigid condition. Any required subsequent repositioning can beaccomplished in the same manner.

Although the stent of the present invention is generally tubular inconfiguration since the vessels in which the stent is normally placedare generally tubular, it must be recognized that different portions ofthese vessels may be sized differently, and therefore the stent must besized and shaped accordingly. For example, above the renal arteries, theaorta changes size, and therefore a variable sized stent designed toaccommodate these changes in shape is required. Referring to FIG. 10, astent 24 which is formed to fit within a vessel 26 having a variablediameter is illustrated. In its expanded condition; the diameter of thestent increases from a small diameter end 28 to a large diameter end 30.It is desirable to form a stent having a variable outer configurationwhich will still apply substantially equal outward radial force to thewalls of the vessel 26 of varying diameter. As previously indicated, astent of this type can be formed in several different ways. First, thesize of the cells 18 can be varied so that the cell size progressivelyincreases from the small diameter end 28 to the large diameter end 30 asshown in FIG. 10. Thus, the larger cells can be formed to expand thestent to a greater diameter than the smaller cells.

In addition to varying cell size, it is possible to vary the outwardradial force of expansion along the length of the stent either incombination with a variation in cell size or with cells of substantiallythe same size. For example, referring to FIG. 11, a stent 32 isillustrated in expanded condition within a vessel, such as a renalartery 34. The cells 36 at the ends of the stent 32 are defined byNitinol wire having a thickness or diameter which is much less than thatof the cells 38 in the central portion of the stent. Therefore, whenexpanded, the cells 38, due to the heavier wire, will provide a greaterradial force on a artery 34 then will the cells 36 at either end of thestent 32.

It should also be noted that the finger portions at one end of the stentare flared outwardly at 40 when the stent is expanded. This isadvantageous for a stent which, for example, is positioned within arenal artery 34, for the flared finger portions 40 will then engage thewalls of the aorta 42. When one end of the stent is so flared, the stentwill not extend outwardly into the aorta, causing an obstruction, forthis is particularly disadvantageous if a second stent, a catheter, orsome other device is to be implanted or moved within the aorta.Secondly, the flared fingers 40 tend to draw back the wall of the aortaas the stent expands opening both the aorta and the renal artery 34.Obviously, the end cells 36 of the stent could be formed to provide theflared portion 40 if no finger portions are included in the stentdesign.

Another method for varying the radial outward force applied by the stentwould be to anneal sections of the stent at different temperatures sothat the temperature transformation level at which various sections ofthis stent fully expand will vary. For example, the central cells of thestent 32 could be annealed at a temperature which would cause thecentral section of the stent to fully expand at normal body temperature.The end cells 36 of the stent could be annealed in such a manner thatthese end cells would fully expand at temperatures slightly higher thannormal body temperature, and consequently, at normal body temperaturealthough these end cells would expand, they would not expand to providefull radial force on the vessel 34. Thus, the radial force provided bythe end cells 36 would be less than that provided by the center cells 38at normal body temperature.

The cell expansion characteristics of a stent may be altered by any ofthe methods described or by the combination of these methods. Thus, cellsize could be varied in combination with either a variation in wire sizeor a variation in annealed temperature, or alternatively, cell sizecould be maintained constant and a variation in wire size could becombined with the variation in annealed temperature. Ideally, for moststent applications, the central portion of the stent will provide agreater radial force on a vessel than the end portions of the stent,although the end portions should expand sufficiently to anchor the stentin place as a catheter carrying the stent is removed.

Referring now to FIG. 12, the elastomeric, polymeric sleeve 22illustrated in FIG. 6 may be formed to an extended length so that it cancontain a plurality of separate stent sections, two of which are shownat 44 and 46 in FIG. 12. The elongated elastomeric sleeve may be cutapart along cut lines, as indicated at 48, between the stent sections,so that a plurality of stents encased within a sleeve as shown in FIG. 6may be obtained from the elongated unit of FIG. 12. Alternatively, asingle elongated stent could be provided within the sleeve 22, andsections of the stent and sleeve could then be cut to various desiredlengths.

In forming the stent of the present invention, it is important toprovide a strong bond between cells along the straight portions 6. Toaccomplish this, the cells of the skeletal frame are first formed ofthermal memory wire which has not yet been annealed to achieve its shapememory form and set the expansion shape of the device, and the straightportions 6 are joined by fusion welding, such as by laser welding, tomelt together the metal of adjacent straight portions. These welds areinitially quite brittle, and consequently, must be subjected to furtherheat which provides grain growth and forms a ductal weld sufficient tobond the cells firmly together during the expansion of the stent againsta vessel wall. Subsequent to the welding process, the stent in theexpanded configuration is annealed at a temperature which is sufficientboth to set the shape of the stent which will be provided in response tothermal memory and also to render the welds ductal. After this heattreatment, the stent is quenched and is ready for use.

The mechanical behavior of the Nitinol material used to form the stentof the present invention may be enhanced in accordance with the presentinvention by limiting the twenty three possible martensite variants forthe material to a much smaller number and preferentially orienting thesevariants and/or the grain structure of the alloy in a specific directiondependent on the design requirements of the ultimate device to be formedfrom the alloy. This orientation can be accomplished with the Nitinol ineither its shape memory form or in its superelastic state. To orient themartensitic variants and/or the grain structure of Nitinol, either thewire or the stent being manufactured is subjected to a magnetic fieldoriented to induce the martensitic variants and/or grain structure toform along a predetermined path which will enhance the mechanicalperformance of the unit. This process may be carried out at varioustemperatures depending on the desired effect, and is applicable tobinary NiTi alloys but may be even more effective in ternary alloys suchas Nitinol with iron (NiTiFe). For the stent of the present invention,the martensitic variants and/or grain structure of the wire is orientedmagnetically at an angle to the longitudinal axis of the wire with apreferred angle being ninety degrees. To achieve this orientation,conventional magnetic orientation processes and apparatus can be used,such as those used in Alinco permanent magnet technology.

Referring to FIG. 13, in accordance with the method of the presentinvention, the thermal memory wire for the stent is wound about pins 50which project from the surface of a metal heat conducting mandrel 52. Itwill be noted that for each cell 18, four pins 50 are provided, with apin being positioned at each end of a straight portion 6 for the cellwhere the straight portion meets the cell end wall 21. Thus, the pinsdefine the extent of the straight portions for each cell. When the stentincludes finger portions 16, additional pins are positioned around themandrel 52 at the ends of the stent to form these finger portions.

In the formation of the mandrel 52, the mandrel is placed in a jig andthe holes for the pins 50 are drilled in the mandrel in accordance witha position program in the central processing unit 54 of a computercontrolled drilling unit 56. The pins are then inserted in the mandrel,and once they receive the wire 4, the program in the central processingunit controls a laser welder 58. Since the pins determine the extent ofthe straight portions 6 to be welded, the laser welder 58 may becontrolled by the program in the central processing unit which programsthe location of the pins so that the welder creates a weld between thepins at the ends of the straight portions 6. It is possible to configurethe jig to allow for the laser beam to be focused through the jig on oneside to the weld zone on the inside (luminal side) of the device. Thusfusion welds can be created on both the outer surface and the innersurface of the device. Once all of the welds are formed, the mandrel isplaced in an annealing oven which heats both the mandrel 52, the welds,and the wire 4 to set the expanded memory configuration of the stent.Then the mandrel is removed and quenched, and this cooling of themandrel and the wire 4 causes the stent to become flexible and expandsufficiently to be removed over the pins 50 of the mandrel 52.

Most stents are delivered following the insertion of a guide wire andcatheter into the obstructed structure, such as an artery or othervessel. The catheter serves to guide the insertion of the stent andprotect the stent from being displaced as it is being pushed. Once thestent is properly positioned, the catheter is usually pulled back overthe top of the stent so that the stent can then be expanded. Normally,the distal end of the device is exposed first during the deliveryprocedure.

There are instances, however, where the preferred delivery of stentdevices is proximal end first. This is especially true when attemptingto accurately place stents in the ostium or mouth of a tubularstructure, e.g. the renal artery at the aorta as shown in FIG. 11. Ifthe proximal end of the stent is flared outward and the proximal end ofthe stent can be delivered first, then it is possible to place the stentin perfect apposition to the ostium where the proximal end of the deviceis anchored and the distal portion of the stent is delivered last. Thiseliminates the usual need to leave some portion of the stent deviceprotruding into the lumen of the aorta.

Referring to FIG. 14, a novel two piece stent delivery unit indicatedgenerally at 60 is shown for placing a stent within a vessel so that theproximal end of the stent is the first end to be expanded and anchored.This delivery unit includes a dilator section 62 and a sheath section 64which is an open ended, elongate tube that slides onto the dilatorsection. The dilator section includes a central tubular body having anenlarged portion 66 with an outer diameter that is sized to be slightlysmaller than the inner diameter of the sheath section 64 so that thesheath section will slide relative to the central tubular body. Thiscentral tubular body also includes a portion of reduced diameter 68which extends outwardly from the enlarged portion to a tapered dilatortip 70. Extending rearwardly from the dilator tip, spaced from andconcentric with the portion 68, is an outer tube 72 which terminates inspaced relationship to the enlarged portion 66. The outer tube has anouter diameter which is substantially equal to the outer diameter of theenlarged portion 66 and is adapted to slidably receive the sheathsection 64. The outer tube defines an annular space 74 between the outertube and the portion 68 of the central tubular body. A centrallongitudinal passage 76 which is open at both ends extends completelythrough the dilator section 62.

In the operation of the stent delivery unit 60 as shown by FIGS.15A-15E, a stent, such the flanged stent 32 of FIG. 11 (shown in brokenlines) is inserted into the space 74 between the outer tube 72 and thereduced portion 68 with the distal end of the stent positioned adjacentto the dilator tip 70. The proximal end of the stent with the flangedportion 40 is positioned outside the space 74 between the outer tube 72and the enlarged portion 66 of the central tubular body. The sheathsection 64 is then moved into place over the enlarged portion 66 and theouter tube 72 to enclose the proximal end of the stent as shown in FIG.15A. With the stent so enclosed, a guidance wire 78 may be insertedthrough the central longitudinal passage 76 to aid in guiding the stentdelivery unit into place in a vessel, such as the renal artery 34.

With the stent 32 properly located, the sheath section 64 is drawn backaway from the outer tube 72 as shown in FIG. 15B to expose the proximalend of the stent. This proximal end, when subjected to normal bodytemperature, now expands into contact with the vessel to anchor thestent in place. When a flared stent 32 is used for the treatment ofostial type lesions, the stent would be located at the ostium and heldin place by contact between the flared portion 40 and a parentstructure, such as the wall of the aorta 42.

Once the proximal end of the stent is anchored in place, the dilatorsection 62 is moved within the sheath section 64 into the vessel 34, asshown in FIGS. 15C and 15D, until the entire stent is exposed and thedistal end is released from the space 74. The entire stent is nowsubjected to body temperature and is designed to expand against thevessel 34 to reach an inner diameter which is greater than the outerdiameter of the stent delivery unit 60. Now, with the stent 32 expandedwithin the vessel 34, the stent delivery unit is withdrawn through theexpanded stent as shown in FIG. 15E.

It is to be understood that the present invention is by no means limitedto the particular constructions herein disclosed and/or shown in thedrawings, but also comprises any modifications or equivalents within thescope of the claims. For example, while the use of the stent has beenillustrated in connection with the vascular system, it will be apparentto those skilled in the art that the stent herein shown and describedfinds equal utility in other bodily passageways.

We claim:
 1. A stent for insertion in a body vessel comprising anelongate body member having a longitudinal axis with a skeletal frameformed to define an elongate chamber which extends through said bodymember, the skeletal frame being formed to assume a first collapsedconfiguration toward said longitudinal axis for insertion in said bodyvessel and to expand radially outward from said collapsed configurationto a second expanded configuration to contact and apply radial force tosaid body vessel, said skeletal frame further being formed to define aplurality of interconnected open cells with each of said cells includingtwo substantially parallel, spaced side walls which are substantiallyparallel to said longitudinal axis in both the first collapsedconfiguration and the second expanded configuration of said skeletalframe and end walls extending between said sidewalls at an angle to saidlongitudinal axis, said cells being arranged around said elongatechamber with sidewalls of adjacent cells arranged in adjacentcoextensive relationship, said cells joined together by an attachmentconnecting adjacent, coextensive cell sidewalls, this being the onlyconnection between said cells, said skeletal frame further including afirst area including cells which are designed differently from cells inat least a second area of said skeletal frame to cause different radialforces to be applied to said body vessel by said first and second areasin the expanded configuration of said skeletal frame.
 2. The stentaccording to claim 1 wherein said cells in said first area of saidskeletal frame are larger than the cells in said second area of theskeletal frame.
 3. The stent according to claim 1 wherein the endwallsof the cells in said first area of the skeletal frame are formed at agreater angle to the longitudinal axis of said body member than are theendwalls of the cells in the second area of said skeletal frame.
 4. Astent comprising an elongate body member having a longitudinal axis anda skeletal frame formed to define an elongate chamber which extendsthrough said body member, the body member being formed of thermal shapememory material which defines a plurality of interconnected, open cellsforming the skeletal frame of said body member, each of said cellsincluding two substantially parallel, spaced, straight side portionswhich are substantially parallel to said longitudinal axis and end wallmeans extending between said side portions at an angle to saidlongitudinal axis, said cells being joined together only along thelengths of said straight side portions, the shape memory material havinga temperature transformation level above which said skeletal frameassumes a first expanded configuration relative to said longitudinalaxis and below which said cell end wall means permit the space betweenstraight side portions of said cell to decrease to permit movement ofsaid straight portions toward said longitudinal axis to collapse saidskeletal frame to a second collapsed configuration, said cell end wallmeans operating at a temperature above said temperature transformationlevel when said skeletal frame is in the second collapsed configurationto expand the skeletal frame to the first expanded configuration, andsleeve means disposed around said elongate body and operating to expandwith said skeletal frame when said skeletal frame assumes said firstexpanded configuration and to collapse with said skeletal frame whensaid skeletal frame assumes the second collapsed configuration.
 5. Astent for insertion in a body vessel comprising an elongate body meanshaving a longitudinal axis, a first end, a second end, and an elongatechamber extending through said body means between said first and secondends, the body means being formed of thermal shape memory material whichis relatively pliable at temperatures below a transition temperature topermit said body means to be collapsed toward said longitudinal axis toa collapsed configuration, for insertion in said body vessel, saidthermal shape memory material operating to expand said body meansradially outward from said collapsed configuration toward an expandedmemory configuration in response to said transition temperature tocontact and apply force to said body vessel, said body means operatingat said transition temperature to expand said first end radiallyoutwardly from said longitudinal axis for a greater distance than saidsecond end is radially expanded.
 6. An expandable cellular device forinsertion in a body vessel comprising an elongate skeletal frame formedto define an elongate chamber with a central longitudinal axis, saidskeletal frame having a first end and a second end and being formed toassume a first collapsed configuration toward said longitudinal axiswhen confined for insertion in said body vessel and to expand radiallyoutward from said collapsed configuration to a second expandedconfiguration when released within said body vessel, the skeletal framebeing formed by a plurality of interconnected open cells with the cellsadjacent to the first end of said skeletal frame forming a generallycylindrical end section in the second expanded configuration of saidskeletal frame, the interconnected cells between said generallycylindrical end section and said second end tapering inwardly towardsaid central longitudinal axis to form the skeletal frame with asubstantially conical section in the second expanded configurationterminating at said second end, said cells in said cylindrical endsection being larger in size than the cells in the substantially conicalsection to cause said skeletal frame in said expanded secondconfiguration to taper inwardly toward said second end.
 7. An expandablecellular device for insertion in a body vessel comprising an elongateskeletal frame formed to define an elongate chamber with a centrallongitudinal axis, said skeletal frame having a first end and a secondend and being formed to assume a first collapsed configuration towardsaid longitudinal axis for insertion in said body vessel and to expandradially outward from said collapsed configuration to a second expandedconfiguration, the skeletal frame being formed by a plurality ofinterconnected open cells with the cells adjacent to the first end ofsaid skeletal frame forming a generally cylindrical end section in thesecond expanded configuration of said skeletal frame, the interconnectedcells between said generally cylindrical end section and said second endtapering inwardly toward said central longitudinal axis to form theskeletal frame with a substantially conical section in the secondexpanded configuration terminating at said second end, the cells formingsaid substantially conical section being smaller in size than the cellsforming said generally cylindrical end section.
 8. The expandablecellular device of claim 7 wherein each of said cells in said generallycylindrical end section includes two substantially parallel, spaced,straight side portions which are substantially parallel to said centrallongitudinal axis and end wall means extending between said sideportions at an angle to said central longitudinal axis, said cells insaid generally cylindrical end section being joined together only alongthe lengths of said straight side portions.
 9. An expandable cellulardevice for insertion in a body vessel comprising an elongate skeletalframe formed to define an elongate chamber with a central longitudinalaxis, said skeletal frame having a first end and a second end and beingformed to assume a first collapsed configuration toward saidlongitudinal axis for insertion in said body vessel and to expandradially outward from said collapsed configuration to a second expandedconfiguration, the skeletal frame being formed by a plurality ofinterconnected open cells with the cells adjacent to the first end ofsaid skeletal frame forming a generally cylindrical end section in thesecond expanded configuration of said skeletal frame, the interconnectedcells between said generally cylindrical end section and said second endtapering inwardly toward said central longitudinal axis to form theskeletal frame with a substantially conical section in the secondexpanded configuration terminating at said second end, each of saidcells in said generally cylindrical end section including twosubstantially parallel, spaced, straight side portions which aresubstantially parallel to said central longitudinal axis and end wallmeans extending between said side portions at an angle to said centrallongitudinal axis, said cells in said generally cylindrical end sectionbeing joined together only along the lengths of said straight sideportions, the skeletal frame in said generally cylindrical end sectionbeing formed of thermal shape memory material.
 10. The expandablecellular device of claim 9 wherein the cells forming said substantiallyconical section are smaller in size than the cells forming saidgenerally cylindrical end section.
 11. The expandable cellular device ofclaim 6 wherein said cells in at least said cylindrical end section areformed of thermal shape memory material which is relatively pliable attemperatures below a transition temperature to permit said cylindricalend section to be collapsed toward said longitudinal axis to a collapsedconfiguration for insertion in said body vessel and which expandsradially outward from said collapsed configuration toward an expandedmemory configuration in response to temperatures above said transitiontemperature.
 12. The expandable cellular device of claim 11 wherein allcells in said skeletal frame are formed of said thermal shape memorymaterial.
 13. The expandable cellular device of claim 11 wherein saidcells in said generally cylindrical end section include two spaced,straight side portions and end wall means extending between said sideportions at an angle to said central longitudinal axis, said cells insaid generally cylindrical end section being joined together only alongthe lengths of said straight side portions.